A pixel structure comprises a photo-sensitive element which is responsive to light. The photo-sensitive element can be a pinned photodiode (PPD). FIG. 1 shows a schematic of a four transistor (4T) pixel structure with a pinned photodiode. The pixel is controlled via reset, transfer and row select switches. The photo-sensitive element generates photo-charges in response to incident light. Before readout of the photo-charge, the floating diffusion (FD) is reset through the reset switch. This reset level is read out through the source follower and selection transistor to the column bus. Then, the charges accumulated into the photodiode are transferred through the transfer transistor to the floating diffusion FD. The signal level is again read out through the source follower and selection transistor.
An advantage of the pinned photodiode is that photo-induced charges (photo-charges) are collected during the integration time and that the charges can then be transferred to the floating diffusion FD during readout. This allows a read out technique called correlated double sampling (CDS) that eliminates kTC reset noise from the FD capacitor. Additionally, the buried nature of the pinned photodiode results in low dark current.
However, pinned photodiodes have some disadvantages. A disadvantage of the pinned photodiode is that it has a more restricted dynamic range compared to a conventional photodiode, such as an n-well/p-substrate junction in a three transistor (3T) pixel. The dynamic range is the range, typically expressed as a ratio, between the largest intensity value that can be resolved by the pixel and the smallest intensity value that can be resolved by the pixel. This disadvantage is particularly problematic when pixel size is reduced.
A paper “4.5 μm Pixel Pitch 154 ke—Full Well Capacity CMOS Image Sensor” and US 2008/0237446A1 describe a pixel which resembles a 4T PPD pixel, but with an additional overflow capacitor, called a Lateral Overflow Integration Capacitor (LOFIC), and a switch which connects the LOFIC to the floating diffusion. A large signal causes overflow charges to be transferred from a photodiode to the overflow capacitor during integration.
The present invention seeks to provide an alternative way of improving the dynamic range of a pixel structure.